Recently, dust cores produced through compression molding of soft magnetic powder are being used as electromagnetic parts for use in an alternating current magnetic field, excellent in magnetic property and having a lot of latitude in three-dimensional profiles thereof. For example, as cores (dust cores) for motors or transformers for use at a frequency of around 50 kHz or less, known are those produced by compression-molding of a soft magnetic powder followed by annealing thereof for distortion removal mentioned below for the purpose of reduction in core loss and increase in magnetic flux density thereof (Patent Reference 1). The soft magnetic powder for dust cores is so designed for the purpose of having excellent compressibility and high insulation property that the surface of a pure iron power is coated with an iron oxide, the surface of the iron oxide is coated with an at least one insulating layer selected from oxides, carbonates and sulfates, and the surface of the insulating layer is coated with a silicone resin layer.
Also known is an iron-based soft magnetic powder for a dust core, which is produced by coating the surface of a magnetic powder comprising Fe as a main component, with a first insulating film and further coating the surface with a second insulating film containing oxide particles dispersed therein (Patent Reference 2). Iron cores produced by compression-molding the powder followed by annealing it for distortion removal are described in the same reference.    Patent Reference 1: JP-A 2006-233295    Patent Reference 2: JP-A 2006-5173